In this thesis transport mechanisms in forced and mixed convective flows over complex surfaces are addressed by experiments and numerical simulations. Compared to classical channel flow the surface waviness adds a degree of complexity to the flow by inducing streamline curvature, flow separation and flow reattachment, thus leading to flow situations which are often present in relevant technical and geophysical applications. Measurements are carried out in a wide water channel facility where the bottom surface is heated. Digital particle image velocimetry (PIV) and planar laser induced fluorescence (LIF) are combined to measure simultaneously momentum and scalar fields. The numerical simulations are performed employing a large eddy simulation (LES). Structural information is obtained by performing a proper orthogonal decomposition (POD) of the velocity components to extract the most energetic flow structures, and additionally by calculating turbulence quantities. The major finding is the presence of longitudinal, coherent flow structures which are found for each flow case and which are directly linked to the transport mechanisms for momentum and scalars.